Heath Gill, age 13, of West Des Moines, Iowa, for his question:

HOW DOES A VACCINATION PREVENT DISEASE?

In the late 1700s it was common knowledge that a person could catch smallpox only once. The disease was an ever present horror that often brought disfigurement or death. Many people hoped to catch a light case of the disease, and then be immune to it for the rest of their lives. And then a British physician named Edward Jenner started working on the problem.

Edward Jenner, a British doctor, discovered vaccination as a way of preventing smallpox. His first vaccination, given to an eight year old boy in 1796, opened the medical door to a new conquest of disease. By 1800, vaccinations became accepted as a way to prevent the dread disease of smallpox.

With a vaccination, dead or weakened viruses or bacteria, or their poisons, are introduced into the body. The injection, called a vaccine, makes it possible for the body to develop a resistance to the disease.

Introduction of the vaccine actually allows the body to manufacture a substance called antibodies which fight the effects of the disease viruses. Vaccines have to be strong enough to cause the body to develop this resistance, but they have to be weak enough so they will not bring about a serious disease.

Vaccine comes from the Latin word vacca which means cow. The name comes about because Edward Jenner used an injection of cowpox virus to prevent smallpox.

Today you can receive vaccinations against cholera, diphtheria, German measles, influenza, measles, mumps, whooping cough, bubonic plague, poliomyelitis, rabies, Rocky Mountain spotted fever, tetanus or lockjaw, typhoid and paratyphoid, typhus, yellow fever and, of course, smallpox.

Almost all of the vaccines are injected into the body although the Sabin polio vaccine is taken through the mouth.

Some vaccinations, often called shots, give protection from infection for life or at least for a long period of time, while others provide protection only for a short time. Sometimes it is necessary to give regular booster shots.

Some vaccinations are given routinely to almost every young person. Others, for such diseases as cholera or yellow fever, are given only to those who plan to travel into areas where the diseases are known to exist.

Some vaccinations are given only to people who risk exposure to the disease. Rabies shots, for example, are only given to people who have been bitten by an animal that is suspected of carrying the disease.

Polio vaccinations are given to almost all young people, and as a result of a most efficient campaign, the disease of poliomeylitis has been almost completely wiped out.